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Figure 2 . Contour plot of an emission-excitation matrix of 8.3 X 10 cene in cyclohexane Data were acquired in 0.6 s on a video fluorometer (29) in a 64 X 64 format
a high degree of selective measure ment and, ultimately, specific mea surement. One obvious approach to additional selectivity is to obtain Ii in terms of three parameters
L· = fiKxXmf)
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(2)
where Ρ is an arbitrary parameter se lected from among the many listed in Figure 1. Furthermore, more than three variables could be used in Equa tion 2 to increase the selectivity of the measurement. A number of other parameters could also be included (10) in Figure 1, par ticularly under the miscellaneous cat egory. However, we have necessarily limited our diagram to include widely used parameters and those that are in creasingly being used for selective measurement. It should also be noted that some of the parameters listed in the miscellaneous category are really not characteristic parameters of the molecule in the excited state. How ever, these characteristic properties of the molecule can be combined with excited-state information to increase selectivity. This article discusses multidi mensional luminescence measurement (MLM) and its applications in analyt ical chemistry. The MLM approach involves the simultaneous use of two or more parameters of luminescence for selective measurement. We term this approach multidimensional rather than multiparametric since, in this ar ticle, we will primarily emphasize data
CIRCLE 2 ON READER SERVICE CARD 464 A · ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
5
M anthra
acquisition, display, and processing in a multidimensional format. There are a number of advantages for reduction of data using a multidimensional for mat (9). An example of such an ap proach was presented in a recent Apage article by McGown and Bright (11) in which phase-resolved fluores cence intensity is derived as a function of synchronously scanned wavelengths and of detector phase angle. Our discussion in this article will be divided into the four major classifica tions of spectral measurements, life time measurements, polarization mea surements, and miscellaneous ap proaches, as depicted in Figure 1. The utility of MLM will be demonstrated using one or two examples in each classification. Spectral measurements Theory. The discussion provided here is necessarily brief; either of two useful monographs (12,13) can be consulted for detailed discussions on the general theory of luminescence. Luminescence measurements have had widespread applications in the analytical studies of large, unsatu rated, highly conjugated, organic mol ecules such as polynuclear aromatic compounds (PNAs). Organic mole cules generally have an even number of electrons. Therefore, the absorption and fluorescence transitions occur be tween singlet states. Phosphorescence generally involves an intersystem crossing to a triplet
